I love Prison Break! Most great shows start out conceptually brilliant initially, then fail to end when they should. Lost and Heroes are good examples of that.
PB however, has kept it's flair for the 4th series, which is mighty impressive. I usually stop watching shows when they stagnate. Prison Break works I think because the main protagonists are still central to the plot. The total amount of key figures hasn't spiralled into a crowd of undeveloped characters like Heroes has. The plot has good continuity without a sense of random developments deliberately designed to extend the show. PB DOES have some instances of extensions, but they WORK, and contribute to the original main plot.
BTW, I hope it ends by series 5 maximum. If they do so, despite very high ratings, I'll show my respect by purchasing the entire lot of episodes on DVD. The only bad thing about Prison Break is the lack of humour at times, yes, it's a super-serious plot, but some humour would benefit it. I only laugh at unintended humour, like when Don Self fell from the ladder today, just cracked me up ^_^
Other annoying thing is predictability intra-plot. For example, if performing a task during a 'mission' something ALWAYS goes wrong, no matter what. I would now be very surprised if a certain task was completed without incident. Other than that, excellent TV.